The present invention relates to a method and apparatus for producing plane parallel optical substrates with uniform optical path characteristics; and in particular, to a system for measuring and quantifying the optical path variations over the surface area of a plane parallel optical blank as compared to a data model of the desired result, coupled with automated localized reductions of substrate thickness of the optical blank to match the model""s optical performance.
It is well known that the fundamental principles of Fabry-Perot interferometers provide a means of quantifying the reflective and transmissive characteristics of optical surfaces. A pair of plane parallel surfaces that are at a fixed distance apart, such as the top and bottom surfaces of a layer of transparent material like glass, suspended in air, exhibit constructive and destructive interference when a light source is introduced. When constructive interference occurs, a maximum amount of light is transmitted through the optical surfaces, while a minimum amount of light is reflected. When destructive interference occurs, a minimum of light is transmitted and the maximum is reflected.
There are a number of parameters that determine the reflective and transmissive properties. As incident light is scanned through a range of wavelengths, the transmitted and reflected components both have a sinusoidal variation of intensityxe2x80x94with one being 180 degrees shifted in phase relative to the other. The period or peak to peak spacing of the sinusoidal pattern is not constant, but varies as a function of the wavelength of the light, the separation of the optical surfaces, and the refractive index of the medium between the surfaces.
There have been various attempts to improve optical surfaces and optimize the response of optical surfaces and other precision surfaces such as of semiconductor wafers. The process of spectro-reflectance is well described in the prior art. Spectro-reflectance is a process of directing a light beam at a surface and measuring the intensity of the reflections as a function of wavelength. The index of refraction of the deposited layers of a semiconductor wafer alters the direction of the reflected beam so that the thickness of the layers can be determined by measuring the deviation of the reflected beam path. The following references may provide useful context to the reader for understanding the invention that follows.
A spectro-reflectance process for measuring the thickness and composition of semiconductor wafer layers is described in Weyburne""s U.S. Pat. No. 6,048,742, published Apr. 11, 2000. A light source is directed at the wafer surface at a known angle off normal, and the angles of reflections are recorded at a photodiode array. The subject wafer is situated on a X-Y stage allowing the wafer can be moved in a stepped pattern so that multiple measurement points can be analyzed. The reflectivity data permits mapping and analysis of the wafer layer construction.
In de Groot""s U.S. Pat. No. 5,751,427, published May 12, 1998, a method of measuring the gap between adjacent optical surfaces of two different devices is disclosed. A light source of a wavelength greater than the gap to be measured is directed at an oblique angle towards the first device. The light transmitted through the first device and reflected off the adjacent optical surfaces of the two devices is measured by an intensity detector and a phase detector. Based upon these measurements, the spacing or gap between the surfaces is computed. And, by altering the gap between the surfaces, ""427 processes the data to calculate the index of refraction.
Another process for measuring variations in thickness of an optical element of an etalon is described in Tracy""s U.S. Pat. No. 5,528,370, published Jun. 18, 1996. A light source of a particular wavelength is directed to the etalon via a mirror, and the output is directed to a splitter. As described by Tracy, by way of well known optical phenomena, a Fabry-Perot interference pattern is effected at the etalon by multiple reflections of the narrow-band radiation between the two surfaces of the etalon. A microscope observes the diffracted beam from the sample off the splitter and measures the surface variations. If the flatness and parallelism of the etalon surfaces are perfect, the intensity at the etalon as seen through the microscope will be uniform and depend on the path wavelength and the exact thickness of the etalon. If there is any variation of thickness, due to wedge shape and/or variations in flatness, an optical interference fringe pattern will be seen through the microscope. The fringes will be parallel if the etalon is wedge shaped, and circular or oval if the etalon has a peak or depression in a surface.
A fringe pattern discriminator is disclosed in Tronolone""s U.S. Pat. No. 5,724,137, published Mar. 3, 1998, where a light source interacts with an interferometer through two optical diffraction gratings between which the target object is suspended. The interferogram are recorded by an imaging system. The interferogram encompasses both the object fringes and the interference fringes caused solely by the gratings. By lateral movement of the gratings relative to the object, the system can distinguish between the object fringes and the interference fringes.
A similar type of invention is disclosed in Erickson""s U.S. Pat. No. 5,327,220, published Jul. 5, 1994. In accordance with this invention, the thickness of optical parts is measured by using a light source in conjunction with an interferometer, where the light source is reflected off the optical surfaces, and the imaging system measures the reflected fringes and interference rings. A computer processes the results to calculate the thickness change by observing the slope of the intensity variation. The ""220 patent also discloses rotating the optical component to map the results.
Misaka""s U.S. Pat. No. 5,620,357, published Apr. 15, 1997, describes a polishing method for reducing wafer taper in a single large wafer polishing machine. Wafer thickness is measured and corrections made by a robotic setup for polishing the wafer surface to reduce or remove the taper, by comparing the center of the wafer to the center of the compressive load caused by the taper; the offset providing an indication of the taper characteristics.
What is needed is a precise and effective means of measuring and correcting the variations in optical path characteristics over a pair of plane and parallel optical surfaces. Such a system should facilitate the automated reduction in thickness of the areas of irregular optical path performance to match an ideal optical performance profile. Ideally, the system would have a single controller making the measurements and coordinating the reduction of the selected areas of the optical surfaces, in order to match the optical characteristics to a model performance profile.
Accordingly, it is an object of the invention to provide a method and apparatus for producing optical plates of uniform optical performance and limited optical transmission path surface variation, by repetitively measuring and reducing optical plate blanks to match a data model of the optical performance of an ideal plate or design template having a particular, pre-selected uniform optical transmission path characteristic across its surface.
The optical transmission path characteristic of a light beam at any point on the plate is a function of the thickness of the plate and refractive index of the plate material. Once plate thickness and refractive index are established, a variation in wavelength of the light source alters the intensity of the reflected and transmitted light in a predictable manner. Conversely, application of light of varying wavelength, causing measurable variations in the intensity of reflected or transmitted light, provides a composite index or indicator of plate thickness and refractive index.
A basic formula for interference in a transmission is mxcex=2nd Cos xcex8, where:
m=order of interference
xcex=free space wavelength
n=index of refraction
d=distance between optical surfaces
xcex8=angle of incidence of the light source
Using certain assumptions, namely that xcex8=0 degrees or Cos xcex8=1; and the variation of n with respect to wavelength is constant.
The underlying principle is that if a plane parallel plate, such as a SiO2 plate, is measured by a detector in multiple places, where the only variables are the plate thickness and refractive index and the shift in the position of the interference peak is measured, then the change in the optical path of the plate can be determined.
For example, consider interference peak number m; at a first position, mxcex1=2 n d1, and at a second position, mxcex2=2 n d2.
Solving for the thickness difference;
(2 n d1)/xcex2xe2x88x922 n d2 
d2=(d1xcex2)/xcex1 
xcex94d=d2xe2x88x92d1 
xcex94d=(d1xcex2)/xcex1xe2x88x92d1 
xcex94d=d1(xcex2/xcex1xe2x88x921)
In accordance with the invention, an optical plate blank is subjected to light from a light source from one side for transmission through the two plane parallel surfaces of the plate, the transmission wavelength of which is determinable by any suitable means, and the transmission pass-through intensity of which is measurable by a suitably aligned optical receiver on the other side of the plate.
Quantifiable variations in the optical path transmission performance over the surface area of the plate are determinable by making these wavelength and intensity measurements at different points on the plate, as is further explained below. Accordingly, the plate blank is repositioned laterally with respect to the light beam for successive wavelength and intensity readings so as to provide a sufficiently high density, uniformly distributed grid pattern of optical transmission path intensity and wavelength readings which constitute an initial data matrix or contour map of optical transmission path data for the optical blank.
The exact cause of variations in the transmission path characteristic matrix of the optical blank is irrelevant for the purpose of the invention; it may be a localized difference in plate thickness or variation in the refractive index or combination of both. The actual thickness of the blank is also irrelevant, as is apparent from the previous discussion. For purposes of the invention, the variable selected by the applicants for localized adjustment to the optical transmission characteristic is the difference in plate thickness at each point on the grid. For this reason, the refractive index is assumed to be constant, and calculations fundamental to the invention proceed around this assumption.
The analytical process applied to each set of coordinates of the grid pattern is based on an ideal optical computer model of similar construction, designed in software using an optical modeling program. The computer model represents an optical specification to which the user wants to produce actual optical plates from pre-formed optical blanks of slightly excessive thickness. The known and initially measured parameters of the optical blank under test, with refractive index assumed to be constant, are compared to the computer model to generate an apparent actual thickness, or optical thickness, for each coordinate point of the grid. The sum of this data is a contour map of the optical thickness of the blank. Using this database contour map, an automated localized reduction process such as ion beam milling operation or a surface polishing operation is performed on the optical blank to reduce the thickness of localized areas identified by the comparison, to more closely match all points on the surface of the blank to the optical performance of the computer model. Multiple iterations of the measuring and reduction process are conducted until the optical blank is sufficiently and uniformly close in performance across its entire surface to the computer model.
It is therefore an objective of the invention to provide both process and apparatus for the automated production of devices having plane parallel optical surfaces of desired optical performance, where there are incorporated steps and means for measuring the intensity and wavelength of a light transmitted through the optical surfaces at several points of known spatial orientation, calculating the optical thickness between optical surfaces at each point by comparing the wavelength and intensity measurements to a computer model of desired optical performance while assuming the refractive index of the plane parallel optical surfaces is constant, and reducing the actual thickness between the optical surfaces at selected high points so as to reduce the variation in optical thickness between the optical surfaces.
Still other objects and advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein only a preferred embodiment of the invention is described, simply by way of illustration of the best mode contemplated by us for carrying out the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention.